U.S. patent number 7,235,301 [Application Number 11/127,819] was granted by the patent office on 2007-06-26 for latent heat storage material, latent heat storage unit containing the material, processes for producing the material and the unit and processes for using the material.
This patent grant is currently assigned to SGL Carbon AG. Invention is credited to Jurgen Bacher, Martin Christ, Oswin Ottinger.
United States Patent |
7,235,301 |
Bacher , et al. |
June 26, 2007 |
Latent heat storage material, latent heat storage unit containing
the material, processes for producing the material and the unit and
processes for using the material
Abstract
A latent heat storage material is in the form of a composite
material formed of a phase-change material to store latent heat and
graphite flakes incorporated therein to improve thermal
conductivity. The graphite flakes are distinguished by a high
aspect ratio and a high anisotropy of thermal conductivity. The
volume content of graphite flakes in the latent heat storage
material is between 10 and 40%. The composite material can be
obtained by mixing the components or infiltrating a bed containing
graphite flakes with a liquid phase-change material. The graphite
flakes are preferably aligned during mixing with the phase-change
material by shaking or tamping, etc., so that the thermal
conductivity in the direction that is advantageous for the
individual application is maximized. A latent heat storage unit
containing the material and processes for producing the material
and the unit and processes for using the material, are also
provided.
Inventors: |
Bacher; Jurgen (Wertingen,
DE), Ottinger; Oswin (Meitingen, DE),
Christ; Martin (Wehringen, DE) |
Assignee: |
SGL Carbon AG (Wiesbaden,
DE)
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Family
ID: |
34925039 |
Appl.
No.: |
11/127,819 |
Filed: |
May 12, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050258394 A1 |
Nov 24, 2005 |
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Foreign Application Priority Data
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May 18, 2004 [EP] |
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04011756 |
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Current U.S.
Class: |
428/408 |
Current CPC
Class: |
F28D
20/023 (20130101); C09K 5/063 (20130101); Y10T
428/30 (20150115); Y02E 60/145 (20130101); Y02E
60/14 (20130101) |
Current International
Class: |
B32B
9/00 (20060101) |
Field of
Search: |
;428/408 ;423/445R
;427/448 ;252/70 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2 472 278 |
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Jul 2003 |
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CA |
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196 30 073 |
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Jan 1998 |
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DE |
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102 00 318 |
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Jul 2003 |
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DE |
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1 416 027 |
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May 2004 |
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EP |
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Other References
Jun Fukai et al.: "Thermal conductivity enhancement of energy
storage media using carbon fibers", Energy Conversion &
Management, No. 41, 2000, pp. 1543-1556. cited by other .
Min Xiao et al.: "Preparation and performance of shape stabilized
phase change thermal storage materials with high thermal
conductivity", Energy Conversion and Management, vol. 43, 2002, pp.
103-108. cited by other.
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Primary Examiner: McNeil; Jennifer
Assistant Examiner: Miller; Daniel
Attorney, Agent or Firm: Greenberg; Laurence A. Stemer;
Werner H. Locher; Ralph E.
Claims
We claim:
1. A latent heat storage material, comprising: a phase-change
material having incorporated particles of graphite; at least a part
of said graphite being made up of flakes having a high anisotropy
of thermal conductivity and a high aspect ratio and being formed of
at least one material selected from the group consisting of natural
graphite and anisotropic synthetic graphite, said graphite flakes
having a volume content in the latent heat storage material of
between 10 and 40%.
2. The latent heat storage material according to claim 1, wherein
said thermal conductivity of said graphite particles in various
crystallographic directions differs by a factor of at least 50.
3. The latent heat storage material according to claim 1, wherein
said aspect ratio of said graphite flakes is at least 1:10.
4. The latent heat storage material according to claim 1, wherein
said graphite flakes have an average particle diameter d.sub.50 of
at least 30 .mu.m.
5. The latent heat storage material according to claim 1, wherein
said graphite flakes have a bulk density of between 250 g/l and 700
g/l.
6. The latent heat storage material according to claim 1, wherein
said thermal conductivity of the latent heat storage material in
one spatial direction is at least twice as high as said thermal
conductivity in a perpendicular spatial direction.
7. The latent heat storage material according to claim 1, wherein
particles of expanded graphite and said graphite flakes are
incorporated in said phase-change material.
8. The latent heat storage material according to claim 1, wherein
said phase-change material is at least one phase-change material
having a melting point in a range of from -100.degree. C. to
500.degree. C. and being selected from the group consisting of
paraffins, sugar alcohols, gas hydrates, water, aqueous solutions
of salts, salt hydrates, mixtures of salt hydrates, salts and
eutectic blends of salts, alkali metal hydroxides and mixtures of
several of said aforementioned phase-change materials.
9. The latent heat storage material according to claim 1, wherein
said phase-change material is sodium acetate trihydrate.
10. the latent heat storage material according to claim 1, wherein
said phase-change material is calcium chloride hexahydrate.
11. the latent heat storage material according to claim 1, which
further comprises at least one nucleating agent.
12. A latent heat storage unit, comprising: a latent heat storage
material according to claim 1; said latent heat storage material
having a form selected from the group consisting of a loosely
packed bed and free-flowing granules.
13. A latent heat storage unit, comprising: a molded article
containing the latent heat storage material according to claim
1.
14. A process for the production of a latent heat storage material,
which comprises the following steps: providing a device selected
from the group consisting of a mixer, an extruder and a kneader;
and mixing components of the latent heat storage material according
to claim 1 with the device.
15. A process for the production of a latent heat storage material,
which comprises producing the latent heat storage material
according to claim 1 by the following steps: producing, in a
vessel, a bed of graphite containing graphite flakes; covering the
bed with a layer of liquid phase-change material; infiltrating the
bed with the liquid phase-change material; and solidfying the
phase-change material.
16. The porcess according to claim 15, which further comprises
applying a vacum or overpressure during the infiltrating step.
17. A process for the production of a latent heat storage material,
which comprises: orienting the graphite flakes of the latent heat
storage material according to claim 1 by shaking or tamping.
18. A process for the production of a latent heat storage unit,
which comprises providing the latent heat storage material
according to claim 1 in the latent heat storage unit by the
following steps: providing a heat storage container having heat
exchanger tubes running in a vertical direction and having a space
between the tubes; introducing a graphite bed containing graphite
flakes into the space between the tubes; orienting the graphite
flakes by shaking or tamping; covering the graphite bed with a
layer of a liquid phase-change material; and infiltrating the
graphite bed with the phase-change material.
19. The process according to claim 18, which further comprises
applying a vacum or overpressure during the infiltrating step.
20. A process for the production of a latent heat storage unit,
which comprises producing a molded article from the latent heat
storage material according to claim 1 by a process selected from
the group consisting of injection molding, extrusion and
press-molding.
21. A process for the production of a latent heat storage unit,
which comprises: producing a block of the latent heat storage
material according to claim 1 in which the graphite flakes have an
orientation; and cutting an article from the block along a cutting
plane perpendicular to the orientation of the graphite flakes.
22. A process for temperature and air conditioning control, which
comprises: controlling temperature and air conditioning of rooms,
buildings and motor vehicles for the transport of heat-sensitive
goods, with the latent heat storage material according to claim
1.
23. a process for cooling or storing heat or energy, which
comprises: cooling electronic components or storing heat, solar
energy or process heat produced in industrial processes, with the
latent heat storage material according to claim 1.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a latent heat storage material in the form
of a composite material formed of at least one phase-change
material in which flake-like graphite particles formed of natural
graphite or synthetic graphite having a high aspect ratio and high
anisotropy of thermal conductivity are incorporated in order to
increase thermal conductivity. The invention also relates to a
latent heat storage unit containing the material and to processes
for the production and use thereof.
Phase-change materials are suitable for storing heat energy in the
form of latent heat. Phase-change materials are understood to be
materials that undergo a phase transition when heat is supplied or
removed, e.g. a transition from the solid to the liquid phase
(melting) or from the liquid to the solid phase (solidification) or
a transition between a low-temperature and high-temperature
modification. If heat is supplied to or removed from a phase-change
material, its temperature upon reaching the phase transition point
remains constant until the material is completely transformed. The
heat supplied or removed during the phase transition, which causes
no temperature change in the material, is known as latent heat.
The low thermal conductivity of those materials is disadvantageous
to the practical application of phase-change materials as heat
storage units. As a consequence, charging and discharging of the
heat storage units is a relatively slow process.
The charging and discharging time for latent heat storage units can
be reduced if the phase-change material is incorporated into a
matrix formed of a material having high thermal conductivity. For
example, German Published, Non-Prosecuted Patent Application DE 196
30 073 A1, corresponding to Australian Patent Application 39 41 197
A, proposed that a porous matrix formed of graphite be impregnated
in vacuo with a "solid-liquid" phase-change material in the liquid
phase. Impregnation can be performed through the use of immersion,
vacuum or vacuum-pressure processes.
U.S. Patent Application Publication No. U.S.2002/0016505 A1
proposed adding an auxiliary agent having a high thermal
conductivity to the phase-change material, for example metal or
graphite powder. In particular, in Example 2 of that disclosure it
is stated that 2 g of the phase-change material didodecyl ammonium
chloride are ground together with 2 g of synthetic graphite KS6 and
press-molded to form a molded article. The advantages of that
procedure reside in variable molding through the use of
cost-effective, industrially applicable molding processes, e.g.
granulation or extrusion, and the possibility of processing solid
phase-change materials and phase-change materials with solid
additives, e.g. nucleating agents. Alternatively, use as a bed in a
latent heat storage container provided with heat exchanger profiles
is possible.
In contrast to the graphite matrix of German Published,
Non-Prosecuted Patent Application DE 196 30 073 A1, corresponding
to Australian Patent Application 39 41 197 A, which is impregnated
with the phase-change material, in the mixtures described in U.S.
Patent Application Publication No. U.S. 2002/0016505 A1, the
particles of the heat-conducting auxiliary agent do not form a
conductive framework incorporating the phase-change material. In
the latter case the thermal conductivity is thus necessarily lower.
A considerable disadvantage in the use of metal chips or synthetic
graphite powder as heat-conducting admixtures lies in the fact that
relatively high proportions of the heat-conducting auxiliary agent
are needed for a significant increase in the thermal conductivity
of the latent heat storage material (see the example above from
U.S. Patent Application Publication No. U.S. 2002/0016505 A1). The
energy density of the latent heat storage unit is reduced as a
consequence.
The production of latent heat storage units from composite
materials formed of phase-change materials that pass from the solid
to the liquid phase upon changing phase, such as e.g. paraffin, a
styrene-butadiene-styrene copolymer encapsulating the phase-change
material and thus stabilizing it in its form and a small proportion
of expanded graphite as heat-conducting auxiliary agent, is known
from a publication by Min Xiao et al., entitled Energy Conversion
and Management, Volume 43 (2002) pages 103 to 108. The composition
of the composite material was given as follows: 80 parts by mass of
paraffin, 20 parts by mass of copolymer and 3 to 5 parts by mass of
expanded graphite. The actual proportion by mass of the
heat-storing material is therefore only just under 80%. The
dimensionally stabilizing material contributes little to heat
conduction and nothing to latent heat storage.
Latent heat storage materials with the addition of expanded
graphite as a heat-conducting auxiliary agent are known from
European Patent Application EP 1 416 027 A1, corresponding to U.S.
Patent Application Publication No. U.S. 2004/0084658 A1.
It was established that even with relatively small volume contents
(5% or more) of expanded graphite, a significant increase in
thermal conductivity is obtained. The addition of a dimensionally
stabilizing material was not necessary. The advantages of that
latent heat storage material with an addition of expanded graphite
in comparison to a latent heat storage material which has an equal
volume content of synthetic graphite, can be attributed to the
special features of the nature, structure and morphology of the
expanded graphite.
The crystal structure of the expanded graphite corresponds much
more closely to the ideal graphite layer plane structure than the
structure in the more isotropic particles of most synthetic
graphites. That is why the thermal conductivity of the expanded
graphite is higher.
Other characteristics of the expanded graphite are the low bulk
density and high aspect ratio of the particles. As is known, for
particles with a low packing density and high aspect ratio, the
percolation threshold, i.e. the critical volume content of those
particles in a composite material for the formation of continuous
conductivity paths, is lower than for more densely packed particles
having a lower aspect ratio and the same chemical composition. The
conductivity is thus significantly increased by even relatively
small volume contents of expanded graphite.
Molded articles can be produced from the latent heat storage
material through the use of extrusion, injection molding or
press-molding methods. Alternatively, a loosely packed bed of the
latent heat storage material can be introduced into a container
provided with heat exchanger profiles for the purposes of heat
storage.
The production of expanded graphite and products made from expanded
graphite is known, inter alia, from U.S. Pat. No. 3,404,061.
Graphite intercalation compounds or graphite salts, e.g. graphite
hydrogen sulfate or graphite nitrate, are heated rapidly in order
to produce expanded graphite. The expanded graphite that is
produced in that way is formed of relatively bulky, worm-shaped or
concertina-shaped aggregates. The bulk density of expanded graphite
ranges from 2 to 20 g/l, preferably from 2 to 7 g/l. As a result of
the bulkiness of the particles and the low bulk density, the
conveying and metering of particles of expanded graphite and the
incorporation of expanded graphite into latent heat storage
materials present some technical difficulties. Furthermore, the
cost of producing expanded graphite, due to the large number of
process steps that are needed and the use of energy and chemicals,
is relatively high.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a latent
heat storage material, a latent heat storage unit containing the
material, processes for producing the material and the unit and
processes for using the material, which overcome the
hereinafore-mentioned disadvantages of the heretofore-known
products and processes of this general type and in which the latent
heat storage material has a heat-conducting auxiliary agent that
has advantageous properties similar to those of expanded graphite
but not its disadvantages during production and processing.
With the foregoing and other objects in view there is provided, in
accordance with the invention, a latent heat storage material. The
latent heat storage material comprises a phase-change material
having incorporated particles of graphite. At least a part of the
graphite is made up of flakes having a high anisotropy of thermal
conductivity and a high aspect ratio and being formed of at least
one material selected from the group consisting of natural graphite
and anisotropic synthetic graphite.
Thus the object of the invention is achieved by forming the latent
heat storage material as a composite material formed of a
phase-change material containing incorporated graphite as a
heat-storing auxiliary agent. The graphite acting as the
heat-conducting auxiliary agent contains flakes of natural graphite
or/and a synthetic graphite having a high anisotropy of thermal
conductivity and a high aspect ratio.
Other features, details and advantages of the invention emerge from
the following detailed description of the invention and the
embodiment examples.
In accordance with the present invention a composite material
having a higher thermal conductivity than the pure phase-change
material is obtained by adding a graphite material to the
phase-change material as a heat-conducting auxiliary agent which
contains particles having a layer plane structure which is very
close to the ideal crystal lattice structure of graphite. The ideal
graphite structure is formed of layer planes lying in parallel and
equidistantly on top of one another with a hexagonal configuration
of the carbon atoms. Only weak bonding forces act between the
individual layer planes. As a result of this anisotropic structure
of the graphite, numerous properties of this material are
direction-dependent, for example the thermal and electrical
conductivity in the layer planes is substantially higher than it is
in the direction perpendicular to the layer planes. In accordance
with another feature of the invention, in the graphites that are
used as the heat-conducting auxiliary agent, the thermal
conductivities in the various crystallographic directions differ by
a factor of at least 50.
The graphite that is suitable for the present invention is formed
of crystallites that are aligned with one another and are formed of
individual layer planes with hexagonally disposed carbon atoms.
These crystallites are in the form of flat platelets, scales or
flakes. The term flakes is used below for the purposes of
generalization. In accordance with a further feature of the
invention, the average particle diameter of the graphite flakes
that are suitable for the present invention is at least 30 .mu.m
and preferably no more than 3 mm.
In accordance with an added feature of the invention, such
flake-like particles display a high aspect ratio, i.e. their extent
in the particle plane (length or diameter) is substantially greater
than their extent perpendicular to the particle plane (thickness).
The aspect ratio of a graphite flake is the quotient of the length
or diameter and thickness. Typical values lie in the range of from
10:1 to 100:1. As a comparison: the aspect ratio of a spherical
particle is 1, since its extent is the same in all spatial
directions.
As a result of the anisotropic structure, the thermal conductivity
in the flakes is greater in the direction with the larger particle
extent, in other words in the flake plane, than in the direction
with the smaller particle extent.
Natural graphites in particular display a marked layer plane
structure and orientation of the crystallites. The special
properties vary, however, between the individual geological
deposits. In the case of synthetic graphites produced by
graphitization of carbon materials obtained by liquid-phase or
solid-phase pyrolysis, the anisotropy is usually less strongly
pronounced, and the shape of the particles is closer to the
spherical form. However, there are also some types of synthetic
graphite which display a marked anisotropy, e.g. TIMREX SFG from
Timcal Ltd. (Bodio, Switzerland).
The alignment of the graphite particles is also retained in
composite materials containing such graphite particles, with a
suitable processing mode, so that the anisotropy of the graphite
can be utilized in the use of the corresponding composite material.
In the present invention the composite materials are formed of
graphite particles and phase-change materials, which should have a
high thermal conductivity in the desired direction of heat
transfer. This can be achieved by aligning the graphite flakes when
mixing them with the phase-change material by shaking, tamping or
other suitable measures, so that the thermal conductivity in the
direction that is favorable for the individual application is
maximized.
In accordance with an additional feature of the invention, the
thermal conductivity of the latent heat storage material in one
spatial direction is preferably at least twice as high as that in a
perpendicular spatial direction.
A further deciding criterion for the choice of suitable graphite
materials is the bulk density. On one hand, the bulk density should
not be as low as in expanded graphite, in order to avoid the
problems in conveying, metering, processing and incorporating the
graphite material that are associated with a low bulk density. On
the other hand, in a graphite with a low bulk density a larger
proportion of pores and cavities is available in which the
phase-change material can be incorporated, allowing composite
materials having a higher volume content of phase-change material
to be produced. In accordance with yet another feature of the
invention, natural graphites and anisotropic synthetic graphites
having a bulk density of between 250 g/l and 700 g/l are
suitable.
In accordance with yet a further feature of the invention, the
volume-related graphite content in the composite materials
according to the invention formed of phase-change material and
anisotropic graphite, is 10 to 40%, preferably 15 to 30%. In the
case of composite materials having a phase-change material that
passes to the liquid phase upon changing phase, the composition of
the composite material is preferably conformed to the bulk density
of the graphite that is used. This means that the volume content of
pores and cavities in the graphite in the form of a loosely packed
bed is determined from a comparison between the bulk density and
the theoretical density of the graphite (2.25 g/cm.sup.3), and then
a volume content of phase-change material is added that is
calculated so that the pores and cavities are almost completely
filled. With a composition adjusted in this way, separation
processes, in particular settling of the graphite, when the
phase-change material is in the liquid state, can be largely
avoided. If the graphite content is lower, the liquid phase-change
material and the graphite separate and the graphite particles
settle on the bottom of the vessel. The higher the graphite
content, the higher the viscosity of the mixture.
With the objects of the invention in view, there is also provided a
process for the production of a latent heat storage material. The
process comprises mixing components of the latent heat storage
material with a mixer, an extruder or a kneader. The intimate
mixing of phase-change material and graphite is achieved by using
suitable mixing methods, for example stirring, mixing in a powder
mixer, kneading or granulation.
With the objects of the invention in view, there is additionally
provided a process for the production of a latent heat storage
material. The process comprises producing the latent heat storage
material by producing, in a vessel, a bed of graphite containing
graphite flakes, covering the bed with a layer of liquid
phase-change material, infiltrating the bed with the liquid
phase-change material, and solidifying the phase-change
material.
Phase-change materials having a solid-liquid phase transition are
preferably mixed in the liquid state with the flake-containing
graphite, for example by stirring the graphite into the liquid
phase-change material or by infiltration of the liquid phase-change
material into a graphite bed containing graphite flakes.
Infiltration can be supported by vacuum or pressure. The
possibility of producing the composite material formed of graphite
and phase-change material by infiltration of the phase-change
material into a loosely packed bed of graphite is a decisive
advantage of the present invention in comparison to the use of
expanded graphite. As a result of the very low bulk density it is
technically extremely difficult to infiltrate loosely packed beds
of expanded graphite with a liquid, because high foaming occurs and
the expanded particles float. Moreover, the mechanical stability of
particles of expanded graphite is so low that when a bed is covered
with a layer of the liquid phase-change material prior to
infiltration, both the structure of the bed and the individual
particles are destroyed.
In order to be able to infiltrate expanded graphite with a liquid
phase-change material, it must first be pre-compressed. It is known
from German Published, Non-Prosecuted Patent Application DE 196 30
073 A1, corresponding to Australian Patent Application 39 41 197 A,
for example, that a porous matrix formed of expanded graphite must
be pre-compressed to a density of at least 75 g/l for impregnation
with a phase-change material in the liquid phase.
The composite materials formed of graphite and phase-change
materials according to the invention can be particularly
advantageously produced through the use of compounding processes
known from plastics technology for the production of compounds,
e.g. kneading or granulation. Compounding through the use of an
extruder, for example a twin-screw extruder, is particularly
preferred. The advantage of this process lies in the fact that the
phase-change material is melted. A greater homogeneity can be
achieved than in a powder mixing process through the continuous
incorporation of the graphite into the liquid phase.
In comparison to the use known from the prior art of expanded
graphite as the heat-conducting auxiliary agent for phase-change
materials, the problems associated with conveying, metering,
processing and incorporating materials having a low bulk density
are avoided with the present invention. A further substantial
advantage of the present invention lies in the fact that natural
graphite can be used directly. By contrast, in order to produce
expanded graphite from natural graphite, graphite salts must first
be produced by treatment with concentrated acids and then expanded
by exposure to high temperatures. The present invention enables the
chemicals and heat energy needed for the production of expanded
graphite to be saved, so that the latent heat storage material
obtained is not only less expensive but also displays a more
advantageous ecological balance.
In accordance with yet an added feature of the invention, mixtures
containing graphite flakes and expanded graphite are added to the
phase-change material as the heat-conducting auxiliary agent. By
choosing the ratio of graphite flakes to expanded graphite, the
person skilled in the art can specifically adjust the bulk density
of the graphite in order to achieve as high a thermal conductivity
as possible combined with the lowest possible graphite content in
the latent heat storage material and the best possible
processability of the graphite blend.
In accordance with yet an added feature of the invention, all
phase-change materials that are inert with respect to graphite in
the working temperature range can be used in the latent heat
storage materials according to the invention. The process according
to the invention for the production of latent heat storage units
allows the use of various types of phase-change materials. The
phase change can reside both in a transition between the liquid and
solid phase and in a transition between various solid phases. The
phase transition temperatures of the phase-change materials that
are suitable for the latent heat storage material according to the
invention are in the range from -100.degree. C. to +500.degree. C.
At phase transition temperatures above 500.degree. C., increased
care must therefore be taken to protect the graphite against
oxidative attack from ambient oxygen.
In accordance with yet an added feature of the invention, suitable
phase-change materials are, for example, paraffins, sugar alcohols,
gas hydrates, water, aqueous solutions of salts, salt hydrates,
mixtures of salt hydrates, salts (particularly chlorides and
nitrates) and eutectic blends of salts, alkali metal hydroxides and
mixtures of several of the aforementioned phase-change materials,
for example mixtures of salts and alkali metal hydroxides or of
paraffins and salt hydrates. Typical salt hydrates that are
suitable as a phase-change material are calcium chloride
hexahydrate and sodium acetate trihydrate.
The choice of phase-change material is made according to the
temperature range in which the latent heat storage unit is
used.
In accordance with yet an additional feature of the invention,
auxiliary substances, e.g. nucleating agents, are added to the
phase-change material if necessary, in order to prevent
supercooling during the solidification process. The volume content
of nucleating agent in the latent heat storage material should not
exceed 2%, since the volume content of nucleating agent is at the
expense of the volume content of heat-storing phase-change
material. Nucleating agents that significantly reduce supercooling
of the phase-change material even in a low concentration are
therefore needed. Suitable nucleating agents are substances that
display a similar crystal structure and a similar melting point to
the phase-change material that is used, for example tetrasodium
diphosphate decahydrate for the phase-change material sodium
acetate trihydrate.
With the objects of the invention in view, there is furthermore
provided a latent heat storage unit, comprising the latent heat
storage material having a form selected from the group consisting
of a loosely packed bed and free-flowing granules.
In accordance with yet another feature of the invention, the latent
heat storage materials can be used as a bed or as a molded article.
Various molding processes known inter alia from plastics technology
are suitable for producing molded articles containing the latent
heat storage material according to the invention, for example
press-molding, extrusion and injection molding. A high anisotropy
of thermal conductivity is typical of these molded articles, since
the graphite flakes orient themselves perpendicularly to the
press-molding direction or parallel to the injection or extrusion
direction. The molded articles are used either directly as heat
storage units or as a component of a heat storage device.
In a press-molded sheet made from the heat storage material
according to the invention, the thermal conductivity parallel to
the sheet plane is therefore higher than it is perpendicular to the
sheet plane. The same applies to injection-molded sheets if the
injection point or injection points are located on one or more
edges of the sheet. If, however, a molded article is to be produced
in which the thermal conductivity perpendicular to the plane is
greater than that in the plane, this can be achieved by cutting the
article from a block of the latent heat storage material in which
the graphite flakes are aligned, so that the cut surface and hence
the plane of the cut-off article runs perpendicular to the
orientation of the graphite flakes in the block. For example, the
desired article can be sawn or sliced from a press-molded block of
the latent heat storage material of appropriate dimensions
perpendicular to the press-molding direction or from an extruded
strand of appropriate dimensions perpendicular to the extrusion
direction. A block in which the graphite flakes are aligned can
also be produced by infiltrating a bed containing graphite flakes,
in which the flakes have been aligned by shaking, with a liquid
phase-change material and then allowing this to solidify. Articles
can likewise be cut from a block of this type in such a way that
the cut plane is perpendicular to the orientation of the graphite
flakes.
The anisotropy of thermal conductivity can be utilized in the
structural layout of the latent heat storage unit by preferably
arranging the molded article made from the latent heat storage
material in such a way that the extent with the higher thermal
conductivity lies in the direction of the desired heat transfer. In
other words, it is oriented towards a heat exchanger profile or an
object having a temperature to be controlled.
In the case of applications in which this is not feasible, a bed
formed of the latent heat storage material according to the
invention can alternatively be used, which is introduced into an
environmentally isolated container provided with heat exchanger
profiles. According to this variant of the heat storage unit, the
latent heat storage material is provided as a powdered mixture or
as free-flowing granules.
In accordance with another mode of the invention, if the
phase-change material is in the liquid state, the flake-like
graphite particles can be disposed in such a bed by tamping or
shaking so that they are substantially horizontal.
With the objects of the invention in view, there is also provided a
process for the production of a latent heat storage unit. The
process comprises providing the latent heat storage material in the
latent heat storage unit by providing a heat storage container
having heat exchanger tubes running in a vertical direction and
having a space between the tubes, introducing a graphite bed
containing graphite flakes into the space between the tubes,
orienting the graphite flakes by shaking or tamping, covering the
graphite bed with a layer of a liquid phase-change material, and
infiltrating the graphite bed with the phase-change material.
If vertical heat exchanger tubes are passed through a bed with
graphite flakes oriented in this way, the graphite flakes oriented
perpendicular to the heat exchanger tubes, i.e. directed away from
the tubes, allow an effective supply of the heat from the heat
exchanger tubes into the interior of the heat storage material or
an effective removal of the heat from the interior of the heat
storage material to the tubes. Such a horizontal configuration in
the bed can be achieved more easily with the flake-like particles
of the anisotropic graphite used according to the invention than
with the bulky particles of expanded graphite.
In accordance with a further mode of the invention, the latent heat
storage material can also be produced directly in the container by
filling it with a bed of flake-like graphite, aligning the graphite
flakes horizontally by shaking or tamping and then infiltrating
them with the liquid phase-change material, wherein the
infiltration can be supported with pressure or vacuum. With
expanded graphite as the heat-conducting auxiliary agent, this
method would not be usable because of the difficulties involved in
infiltrating a bed of expanded graphite, as already described.
With the objects of the invention in view, there is concomitantly
provided a process for using the latent heat storage materials
according to the invention in latent heat storage units, for
example for the temperature control and air conditioning of rooms,
buildings and motor vehicles, for example for the transport of
heat-sensitive goods, for cooling electronic components or for
storing heat, in particular solar energy or process heat produced
in industrial processes.
Other features which are considered as characteristic for the
invention are set forth in the appended claims.
Although the invention is described herein as embodied in a latent
heat storage material, a latent heat storage unit containing the
material, processes for producing the material and the unit and
processes for using the material, it is nevertheless not intended
to be limited to the details given, since various modifications and
structural changes may be made therein without departing from the
spirit of the invention and within the scope and range of
equivalents of the claims.
The construction and method of operation of the invention, however,
together with additional objects and advantages thereof will be
best understood from the following description of specific
embodiments when read in connection with the accompanying
examples.
EXAMPLE 1
In order to produce composite materials formed of graphite and
phase-change material in which the volume-related graphite content
corresponds to the volume fraction of the particular graphite in
the graphite bed, the following procedure was used: First of all
the bulk density or compacted bulk density of the graphite to be
used was determined. A bed of the graphite was then produced in a
beaker. The graphite flakes were substantially oriented
horizontally therein. The graphite bed was then covered with a
layer of the liquid phase-change material. The phase-change
material was metered in that case in such a way that its volume
content corresponded to the pore volume in the graphite bed. Under
the influence of gravity the phase-change material flowed into the
pores in the graphite bed and filled them. This process can be made
easier or accelerated by evacuation (vacuum infiltration), the
application of an external gas pressure (pressure infiltration) or
a combination of both procedures (vacuum-pressure infiltration).
Following solidification of the phase-change material a solid
composite is formed, which can be removed from the beaker after
partial melting of the surface, e.g. in a water bath.
As a consequence of the orientation of the graphite flakes, the
composite formed of graphite and phase-change material displays a
higher thermal conductivity in the direction that was horizontal
during infiltration ("horizontal thermal conductivity") than
perpendicular to that direction ("vertical thermal conductivity").
The orientation and the volume content of the graphite in the heat
storage material can be additionally increased by shaking the
graphite bed before infiltration.
Latent heat storage composites were produced using this procedure
from the graphites listed in Table 1 and the phase-change material
paraffin RT54 (from the firm Rubitherm, Germany), which displays a
solidification point of 54.degree. C. Samples were taken from the
cooled graphite-paraffin composites, on which the horizontal
thermal conductivity with the paraffin in the solidified state was
determined.
As a result of the differing bulk densities or compacted bulk
densities, the composite materials produced in this way display
diverging graphite contents. In order to nevertheless be able to
compare the heat-conducting properties of the various composites,
the thermal conductivity was divided by the volume fraction of
graphite in the composite. This value characterizes the
effectiveness of the type of graphite used in each case in terms of
the increase in thermal conductivity achieved. The results are
summarized in Table 2. It was found that based on the volume
content in the paraffin-graphite composite, natural graphites or
anisotropic synthetic graphites result in a significantly higher
increase in the thermal conductivity of the composite than
isotropic synthetic graphites.
TABLE-US-00001 TABLE 1 Average particle Product diameter name
Manufacturer Graphite type (d.sub.50)/[.mu.m] Stratmin 5098 Timcal
Ltd., Natural graphite 385 Switzerland TFL 898 Graphit Kropfmuhl
Natural graphite 230 AG, Germany Luoyang 599 Luoyang Guangi Ind.
Natural graphite 395 & Trade Co., China SFG 150 Timcal Ltd.,
Anisotropic 55 Switzerland synthetic graphite KS 6 Timcal Ltd.,
Isotropic 3.3 Switzerland synthetic graphite KS 150 Timcal Ltd.,
Isotropic 50 Switzerland synthetic graphite Graphitized SGL Carbon
Group Isotropic 1000 coke synthetic graphite
TABLE-US-00002 TABLE 2 Graphite Thermal content/ conductivity/
Effectiveness/ Composite [vol. %] [W/(m K)] [W/(m K vol. %)]
Stratmin 5098/RT54 28 8.4 0.30 TFL 898/RT54 21 6.6 0.31 Luoyang
599/RT54 30 11.5 0.39 SFG 150/RT54 11 2.7 0.24 KS 6/RT54 7.5 1.0
0.13 KS 150/RT54 24 4.4 0.18 Graphitized coke/ 36 4.2 0.12 RT54
EXAMPLE 2
Composites formed of the phase-change material paraffin RT54 and
natural graphite (TFL 898) and isotropic synthetic graphite
(KS150), respectively, with approximately the same graphite
content, were produced by the method described in Example 1. The
horizontal thermal conductivity was determined when the paraffin
had solidified. Despite the somewhat lower graphite content, the
composite with natural graphite displayed a substantially higher
thermal conductivity than the comparative sample with isotropic
synthetic graphite (see Table 3).
TABLE-US-00003 TABLE 3 Graphite content/ Thermal conductivity/
Composite [vol. %] [W/(m K)] TFL 898/RT54 21 6.6 KS 150/RT54 24
4.4
EXAMPLE 3
In order to investigate the influence of the phase-change material
on the thermal conductivity of the composite, composite materials
formed of natural graphite TFL 898 and the phase-change materials
RT54 (from the firm Rubitherm, Germany) or sodium acetate
trihydrate (NaAc*3H.sub.2O, from the firm Silbermann, Germany) were
produced by the method described in Example 1. The horizontal
thermal conductivities of the composite materials and of the pure
phase-change materials are shown in Table 4. The higher thermal
conductivity of the pure NaAc*3H.sub.2O in comparison to the pure
RT54 also leads to a higher conductivity in the
NaAc*3H.sub.2O-graphite composite.
TABLE-US-00004 TABLE 4 Graphite content/ Thermal conductivity/
Composite [vol. %] [W/(m K)] TFL 898/RT54 21 6.6 RT54 0 0.2 TFL
898/NaAc*3H.sub.2O 21 7.7 NaAc*3H.sub.2O 0 0.6
EXAMPLE 4
Table 5 shows the horizontal thermal conductivity of composites
formed of natural graphite (TFL 898) and the paraffin RT54 with
various graphite contents. The composites were produced in the same
way as in Example 1. A higher graphite content leads to a higher
thermal conductivity.
TABLE-US-00005 TABLE 5 Graphite content Thermal conductivity/
Composite [vol. %] [W/(m K)] TFL 898/RT54 21 6.6 TFL 898/RT54 28
10.7
EXAMPLE 5
In order to investigate the anisotropy of thermal conductivity in
graphite-containing latent heat storage materials, composites were
produced from the phase-change material paraffin RT54 and natural
graphites (Stratmin 5098, Luoyang 599) and isotropic synthetic
graphite (KS 150), respectively, by the method described in Example
1. In order to improve the orientation of the flakes, the bed of
Luoyang 599 was shaken before infiltration with paraffin. The
thermal conductivity in the horizontal and vertical direction was
measured in all composite materials. The anisotropy factor A was
determined from the quotient of these two values. The results are
summarized in Table 6. The composites with natural graphite as the
heat-conducting auxiliary agent display significantly higher
thermal conductivities in the horizontal direction and anisotropy
factors than the composite containing isotropic synthetic graphite.
The comparison between the two composite materials containing
natural graphite shows that the composite with horizontally
oriented graphite particles displays a lower vertical thermal
conductivity on one hand but a substantially higher horizontal
thermal conductivity on the other hand. This leads to a markedly
higher anisotropy factor.
TABLE-US-00006 TABLE 6 Graphite Thermal conductivity/ content [W/(m
K)] A Composite [vol. %] Horizontal Vertical [-] Stratmin 5098/RT54
28 8.4 3.8 2.2 Luoyang 599/RT54 30 11.5 2.5 4.6 KS 150/RT54 24 4.4
2.9 1.5
This application claims the priority, under 35 U.S.C. .sctn. 119,
of European Patent Application 04 011 756.6, filed May 18, 2004;
the entire disclosure of the prior application is herewith
incorporated by reference.
* * * * *